01 - Preheat your oven to 375°F (190°C).
02 - In a large oven-safe skillet (preferably cast iron), cook the bacon over medium heat until crisp. Transfer bacon to a paper towel-lined plate and crumble when cool. Drain off all but 1 tablespoon of bacon fat from the skillet.
03 - Add the olive oil to the skillet. Sauté the diced potatoes and onion over medium heat until potatoes are golden and just tender, about 8–10 minutes. Stir occasionally.
04 - In a large bowl, whisk together eggs, milk, salt, and pepper. Stir in Gruyère, Parmesan, chives, and crumbled bacon.
05 - Evenly distribute the potato and onion mixture in the skillet. Pour the egg mixture over the top, gently stirring to combine. Cook over medium-low heat for 3–4 minutes, just until the edges begin to set.
06 - Transfer the skillet to the preheated oven and bake for 15–18 minutes, or until the frittata is puffed and set in the center.
07 - Remove from the oven and let rest for 5 minutes. Garnish with extra chives, slice, and serve warm.